a) Field of the Invention
This invention relates to a spark plug to be mounted on an internal combustion engine, especially to electrodes of the spark plug.
b) Description of the Related Art
As electrodes of a spark plug to be mounted on an internal combustion engine, those made of an alloy composed of 85 to 70 wt. % of Pt and 15 to 30 wt. % of Ir have been practically used to reduce the consumption (wear) of the electrodes through repetitions of a spark discharge to an air-fuel mixture in a combustion chamber of the internal combustion engine and hence to improve its durability Japanese Patent Publication (Kokoku) No. HEI 2-58756!. With a view to achieving further improvements in the durability by minimizing the consumption of a center electrode through repetitions of a spark discharge, many electrodes made of Ir or sintered Ir--Y.sub.2 O.sub.3 alloys have also been proposed and actually used.
During use in an ordinary internal combustion engine, the temperature of an insulator-supported center electrode, for example, at its central free end remains around about 800.degree. C. When the electrode is made as in the conventional art, namely, is made of an alloy composed of 85 to 70 wt. % of Pt and 15 to 30 wt. % of Ir as disclosed in Japanese Patent Publication (Kokoku) No. HEI 2-58756 or is made of Ir or a sintered Ir--Y.sub.2 O.sub.3 alloy, the consumption of the center electrode through repetitions of a spark discharge can be reduced because the alloy material making up the center electrode contains a great deal of Ir which has excellent spark consumption resistance in the temperature range of the central free end of the center electrode. Keeping in step with the recent move toward a higher power output in internal combustion engines, more heat may however be produced as a result of combustion of an air-fuel mixture in a combustion chamber of an internal combustion engine. The temperature of the center electrode at the central free end thereof exposed in the combustion chamber therefore rises considerably. When the temperature becomes as high as about 850.degree. C. or higher, the volatility of IrO.sub.3 which is formed on the center electrode as a result of repetitions of a spark discharge becomes significantly higher than that of PtO.sub.2 formed likewise on the center electrode. In the high temperature range that the temperature of the center electrode is 850.degree. C. and higher at the central free end thereof, vaporization of IrO.sub.3 is more dominant than that of Pt or an alloy composed of 80 wt. % of Pt and 20 wt. % of Ir so that the spark consumption resistance of the center electrode is conversely reduced and the durability of the center electrode is also lowered.
To cope with the above-mentioned problem, it has also been proposed to sinter Ir powder or Ir--Pt mixed powder and to use the sintered product as a discharge electrode of a center electrode Japanese Patent Applications Laid-Open (Kokai) Nos. SHO 61-88479 and SHO 63-257193, etc.!. In a discharge electrode formed by sintering as described above, Ir and Pt are however not alloyed in a uniformly distributed form as illustrated in FIG. 4. As a result, Ir-only portions and Pt-only portions are localized in the discharge electrode. In the high temperature range of 850.degree. C. and higher at the central free end of the center electrode, more dominant vaporization of IrO.sub.3 formed through repetitions of a spark discharge takes place compared with vaporization of PtO.sub.2 formed from Pt or a Pt alloy. This results in nonuniform consumption of the electrode by sparks, leaving only porous Pt behind. There is accordingly a problem that the durability required for electrodes or the like cannot be sufficiently assured.